DIVISION OF VERTEBRATA INTO CLASSES. 



91 



the body, not being in any instance diffused or scattered through- 

 out the whole, as in Insects and some other tribes. In all but 

 the lowest group, namely Fishes, and the lowest tribe of Reptiles 

 during their larva state, the organs of respiration are adapted 

 to breathe air ; but in those, the aquatic respiration, characteristic 

 of the Molluscous classes, is still retained. In the classes in 

 which the respiration is carried on most actively, and in which 

 the whole mass of the blood is exposed to its influence, before again 

 circulating through the system, namely, Mammals and Birds, 

 the animal has the power of maintaining an elevated temperature, 

 independent of that of the surrounding air ; and they are termed 

 warm-Hooded from this circumstance ; those in which the tem- 

 perature of the body varies with that of the surrounding air or 

 water, being cold-blooded. There is further essential distinction 

 among the different classes of Yertebrata, dependent on the mode 

 in which the function of reproduction is performed in them ; this 

 takes place by eggs in the three lowest classes ; but in the highest, 

 that of Mammals, the young are born alive, and are nourished 

 afterwards by suckling. 



70. It is upon the mode in which the functions of Circula- 

 tion, Respiration, and Reproduction, are performed, in the different 

 groups of Vertebrata, that the division of them into classes is 

 founded; and the following table will show, at a glance, the 

 principles on which this is accomplished ; besides furnishing some 

 other characters, which have not been here adverted to, but 

 which will be hereafter considered in detail, under their respec- 

 tive heads : 



* Oval in a few exceptions. 



